The present invention relates to fuel cells, in particular PEM fuel cells in which a solid polymer is used as an electrolyte.
Fuel cells convert a fuel and an oxidation agent in separate locations on two electrodes into current, heat and water. The fuel can be hydrogen or a hydrogen-rich gas, the oxidation agent oxygen or air. The process of energy conversion in the fuel cell is characterized by a particularly high efficiency. For this reason, fuel cells in combination with electric motors are acquiring increasing importance as alternatives for conventional internal-combustion engines.
On account of its compact design, its power density and its high efficiency, the so-called polymer-electrolyte fuel cell (PEM fuel cell) is suitable for use as an energy converter in motor vehicles.
The PEM fuel cell consists of a stack of membrane-electrode units (MEE) between which bipolar plates are disposed for gas supply and current conduction. A membrane-electrode unit consists of a polymer-electrolyte membrane that is provided on both sides with reaction layers, the electrodes. One of the reaction layers is devised as an anode for the oxidation of hydrogen and the second reaction layer as a cathode for the reduction of oxygen. So-called gas distribution structures made of carbon fiber paper or carbon cloth are applied to the electrodes that permit the reaction gases good access to the electrodes and a good conductance of the cell current. Anode and cathode contain so-called electrocatalysts that catalytically support the appropriate reaction (oxidation of hydrogen or reduction of oxygen). Preferred catalytically active components are the metals of the platinum group of the Periodic System of the Elements. In the main, so-called supported catalysts are used in which the catalytically active platinum group metals are deposited in highly disperse form on the surface of a conductive support material. The mean crystallite size of the platinum group metal used in this situation is about between 1 and 10 nm. Fine-particulate carbon blacks have proved effective as support materials.
The polymer-electrolyte membrane consists of proton-conducting polymer materials. These materials are hereinafter also referred to in abbreviated form as ionomers. A tetrafluoroethylene-flurovinylether copolymer with acid functions, in particular sulfonic acid groups, is advantageously used. A material of this kind is for example sold under the trade name NAFION.RTM. by E. I. du Pont. It is, however also possible to use other, in particular fluorine-free, ionomer materials, such as sulfonated polyether ketones or aryl ketones or polybenzinidazoles.
U.S. Pat. No. 4,876,115 describes a process for the treatment of a porous gas diffusion electrode that has a catalyst loading of less than 0.5 mg/cm.sup.2 on carbon particles. The electrode is impregnated with a solution of a proton-conducting material. This coats the surfaces of the carbon particles with the proton-conducting material.
U.S. Pat. No. 5,234,777 proposes a membrane-electrode unit that consists of a polymer-electrolyte membrane and a composite layer of a supported platinum catalyst and an ionomer. This layer is characterized in that it is less than 10 .mu.m thick and the supported platinum catalyst is evenly dispersed in the proton-conducting ionomer. The platinum loading of the electrodes is less than 0.35 mg/cm.sup.2. The electrode layers are attached to the polymer-electrolyte membrane.
Various processes are described to prepare the membrane-electrode units according to U.S. Pat. No. 5,234,777. In one embodiment, the Pt/C supported catalyst is dispersed in an alcoholic solution of the ionomer. This dispersion, also termed "ink," is applied to a PTFE carrier foil (PTFE: polytetrafluorethylene), dried and laminated by hot pressing onto the opposing sides of a polymer-electrolyte membrane.
In another embodiment the polymer-electrolyte membrane is directly coated with an ink of a Pt/C supported catalyst and a solution of an ionomer. The applied layer is dried at least at 150.degree. C.
The reaction layers according to U.S. Pat. No. 5,234,777 are characterized by a homogeneous distribution of the catalyst in the ionomer. The hot pressing produces dense and pore-free layers of less than 10 .mu.m, advantageously 5 .mu.m thickness, with platinum loadings of less than 0.35 mg Pt/cm.sup.2. In the case of the membrane-electrode units according to U.S. Pat. No. 5,234,777, access of the reaction gases to the catalyst is limited because of the dense, pore-free reaction layer. This has a negative effect on the electrochemical performance of the PEM cell, in particular during operation with dilute gases such as air or reformed gas. The possible use of air and reformed gas in place of oxygen and hydrogen is, however, an important prerequisite for the economic use of fuel cells in motor vehicles.
Another disadvantage of the process described in U.S. Pat. No. 5,234,777 is the high drying temperature of at least 150.degree. C. Under these conditions, solvent vapors can ignite at the catalyst layer and destroy the membrane-electrode unit.
DE 196 02 629 A1 proposes a process for the preparation of a membrane-electrode unit in which a precious metal catalyst on carbon support is used to which the ionomer is adsorbed as colloid. For this purpose a colloid solution of the isomer is prepared in a suitable organic solvent and the carrier catalyst treated therewith. The carrier catalyst coated with the colloid is worked into an ink and used to prepare an electrode that is pressed onto the polymer-electrolyte membrane.
The membrane-electrode units prepared according to DE 196 02 629 A1 do not, however, display improved access for the reaction gases to the catalyst. Furthermore, it is very difficult to achieve a defined and reproducible distribution of the ionomer in colloidal form on the supported catalyst. The stability of the colloid ionomer is limited. The transfer of the process into mass production is therefore only possible to a limited extent.
EP 0 797 265 A1 describes a membrane-electrode unit for PEM fuel cells with a high total porosity and improved electrochemical performance. The high porosity is achieved by using pore formers in combination with a special spray process. The process has the disadvantage that the pore former leads to contamination and additional steps are needed to remove the pore former from the membrane-electrode unit.
The widespread commercial use of PEM fuel cells in motor vehicles calls for further improvement in the electrochemical cell performance and a marked reduction in system costs that are largely caused by the platinum group metals needed. To reduce the costs per kilowatt of installed performance it is therefore necessary to reduce the loading of the electrodes of a PEM fuel cells with the platinum group metals. For this purpose it is necessary to further improve the electrocatalysts and to utilize them more effectively.
It was therefore the object of the present invention to provide an improved membrane-electrode unit and processes for their preparation that avoid the described disadvantages of the state of the art. In particular it was an object to simplify gas transport in the reaction layer and thus to permit improved access of the reaction gases to the catalyst.